Mode locking in semiconductor lasers by phase-conjugate optical feedback

We show theoretically through computer simulations that phase-conjugate optical feedback that is realized through four-wave mixing can produce mode locking in multilongitudinal-mode semiconductor lasers. Phase-conjugate feedback, in contrast to ordinary optical feedback, directly couples pairs of longitudinal modes. For certain strengths of phase-conjugate reflectivities, the phases and beat frequencies of various longitudinal modes become locked in such a way that the laser produces ultrashort mode-locked pulses, even though the laser is pumped continuously.